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Development of novel electroactive nanofluids for flow cells
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[eng] Flow cells are on their way to become a key player for electrical energy storage (EES) thanks to their suitability as load-levelling devices thus contributing to the development of smart grid and to offset the intermittency of renewable energy sources. Until recently flow cells have been limited to Redox Flow Batteries (RFB), where energy storage is given by the redox reactions of dissolved ions. Very recently, new types of “flowable” electrodes have been proposed making use of capacitive storage mechanism (Electrochemical Flow Capacitors or EFCs). Our group has been one of the pioneering labs in this type of novel flow cells based on electroactive nanofluids. The present thesis aimed at harnessing the activity of well-known electroactive species (quinones, graphene, polyoxometalates, LiFePO4) in novel electroactive nanofluids. An important part of our strategy has been the design of hybrid formulations and systems which could combine faradaic (redox) and capacitive (double-layer) storage mechanisms in order to improve the performance of the resulting flow cells. We make an extended review and perspective of the electrochemical flow cell technology and their possible lines of evolution in the introduction of this thesis. With this we introduce the state of the art, the issues to solve and the different solutions proposed. Moreover, we also show our point of view and prospective for this technology and electrical energy storage in general. In the chapter 4, the electrochemical fundamentals of quinones in lithium-organic electrolytes are studied. Quinones electrochemical mechanisms have been widely studied in aqueous media. In this work we study them in an organic electrolyte in an attempt to take advantage of the greater solubility and wider potential windows available in this media. We found and describe in detail several issues preventing the reversible functioning of quinones in Li+ organic electrolytes which in turn preclude their use in flow cells under those conditions. Chapter 5 describes the synthesis, characterization and electrochemical performance of hybrid materials based on reduced graphene oxide (rGO) and polyoxometalates dispersed in an aqueous H2SO4 electrolyte in order to produce a nanofluid. These nanofluids feature low viscosity and show an ultrafast electrochemical response. We demonstrated their functioning as energy storage fluids with full charge and discharge of all solid material dispersed. In chapter 6 a new kind of rGO nanofluid is presented. Instead of using conventional surfactants, we dissolved an aromatic molecule able to stabilize rGO in an aqueous electrolyte. With this approach we achieved a great increase in the stability of the nanofluid. Furthermore, this new nanofluid also showed a great charge transfer capability, as demonstrated by its enabling of the redox activity of LiFePO4 nanoparticles. Thus, thanks to the presence of rGO in the nanofluid, electrons could reach the dispersed nanoparticles and thus be effectively and fully charged and discharged, something not possible in nanofluids containing only LiFePO4 nanoparticles. Graphene synthesis has been also deeply studied as part of this thesis as is shown in the chapter 7. As a result, a new method for the production of graphene by electrochemical exfoliation of graphite has been developed and patented. The patent, a summary of the results obtained and the state of the art of the electrochemical exfoliation method of graphene are presented in this thesis as the last chapter describing research work carried out within the framework of this thesis.
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RUEDA GARCÍA, Daniel. Development of novel electroactive nanofluids for flow cells. [consulta: 6 de desembre de 2025]. [Disponible a: https://hdl.handle.net/2445/174288]